Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Circuit/System Verification
- Ignition ON/Vehicle In Service Mode.
- Verify two or more devices are not communicating on the chassis high speed GMLAN serial data circuit. Refer to Data Link References to determine how many devices should be communicating on this bus.
- If only one device is not communicating
Refer to Circuit/System Testing - Testing the Device Circuits.
- Go to next step: If two or more devices are not communicating
- If only one device is not communicating
- Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down. Disconnect the scan tool from the X84 Data Link Connector. The following tests will be done at the X84 Data Link Connector.
- Test for less than 10 Ω between the ground circuit terminal 5 and ground.
- If 10 Ω or greater
- Ignition/Vehicle OFF.
- Test for less than 2 Ω in the ground circuit end to end.
- If 2 Ω or greater, repair the open/high resistance in the circuit.
- If less than 2 Ω, repair the open/high resistance in the ground connection.
- Go to next step: If less than 10 Ω
- Ignition ON/Vehicle In Service Mode.
- Test for less than 4.5 V between the serial data circuits listed below and ground.
- Terminal 12
- Terminal 13
- If 4.5 V or greater
Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short to Voltage.
- Go to next step: If less than 4.5 V
- Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
- Test for greater than 100 Ω between the serial data circuits listed below and ground.
- Terminal 12
- Terminal 13
- If 100 Ω or less
Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short to Ground.
- Go to next step: If greater than 100 Ω
- Test for 50 - 70 Ω between the serial data circuit terminals 12 and 13.
- If less than 35 Ω
Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short between the Circuits.
- If between 35 - 50 Ω
There may be a third terminating resistor between the serial data circuits. This can happen if the incorrect device is installed. Some devices are available with and without the terminating resistors installed to reduce the need of terminating resistors in the wiring harness. Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short between the Circuits.
- If greater than 70 Ω but less than infinite
Refer to Circuit/System Testing - Testing the Serial Data Circuits for an Open/High Resistance.
- If infinite resistance
Repair the open/high resistance in the circuit between the X84 Data Link Connector and the first splice/device in the serial data circuit.
- Go to next step: If between 50 - 70 Ω
- If less than 35 Ω
- Refer to Circuit/System Testing - Testing the Device Circuits.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.